(a) Field of the Invention
The present invention relates generally to a method for steering a satellite antenna beam, and more particularly, to a method for simplifying the steering of an antenna beam on a satellite in an inclined earth orbit in order to compensate for cross-track motion of earth-based terminals that is caused by rotation of the Earth.
(b) Description of Related Art
Antenna systems for communication satellites that are in non-geostationary orbits may require continuous adjustment of beam steering directions relative to the satellite to maintain coverage of users located within an earth-fixed cell during the pass of the satellite over the cell. The direction from the satellite to the users in satellite coordinates is affected by the rotation of the Earth as well as by the orbital motion of the satellite. The surface speed of the Earth due to rotation is proportional to the cosine of the latitude of the satellite, which varies throughout the orbit for all but equatorial (zero-inclination) orbits. This variation of relative velocity as a function of latitude normally requires beam steering in the cross-track direction (i.e., orthogonal to the velocity vector of the satellite) as well as in the along-track direction (i.e., along the velocity vector of the satellite), which in turn results in excessively complicated and cumbersome beam steering systems.
If the satellite antenna system is an electronically steered, high gain, low side lobe multibeam array, antenna steering may involve the control of the phase and amplitude of many elements. The number of active control elements required is substantially increased when beam steering is required in the cross-track direction as well as the along-track direction. This is normally the case, since for an antenna array aligned with the satellite geometric axes, cross-track motion results from the rotation of the Earth.